a. Field of the Invention
Broadly speaking, this invention relates to the measurement of distortion. More particularly, in a preferred embodiment, this invention relates to methods and apparatus for measuring the intermodulation distortion generated by microwave repeaters, and the like.
B. Discussion of the Prior Art
Multi-channel, microwave transmission systems employing suppressed-carrier, single-sideband, amplitude-modulation have been proposed as a replacement for existing frequency-modulation systems. Due to the nature of amplitude-modulation, such systems will be more sensitive to non-linearities in the microwave repeaters than are the FM systems, for example. Of particular concern are third order IM distortion products which tend to fall within the passband of the transmission system and, hence, are potentially troublesome. Accordingly, whenever a single-sideband microwave route is placed in service it will be necessary to implement a testing procedure providing for routine field measurements of the linearity of the microwave repeaters along the route to insure that distortion of the amplitude-modulated signal is held to a minimum.
One way to perform this routine maintenance procedure would be to selectively take each repeater along the route out of service, and then subject the repeater to any of several, known IM testing procedures. Alternatively, the novel, swept-frequency testing procedure described in my co-pending patent application, H. Miedema U.S. Pat. No. 4,048,559 issued Sept. 13, 1977, may be employed.
All such testing procedures require that the repeater under test be taken out of service, which complicates scheduling of the test procedure. Also, the generation and insertion of multiple test-tones, and the selection of the desired intermodulation products, requires complicated and expensive hardware. Further, the high-level test-tones employed create a potential interference problem in adjacent systems.
Clearly, what is needed is a testing procedure which can be performed on a repeater in situ, and preferably while the repeater is in service, under actual working conditions. This testing procedure will advantageously be less complicated, and the testing equipment less expensive, than that required heretofore.